The lung epithelium has evolved to serve a number of functions ranging from gas exchange in the alveolus to the regulation of mucus clearance in the larger conducting airways. The heterogeneous mix of epithelial cell types enables these functions at the different levels of the airway. Airway epithelial cells include mucin producing and secreting goblet cells, which provide a mucus gel for the multi-ciliated cells to propel out of the airways, surfactant producing type 2 pneumocytes, which maintain alveolar patency with type 1 pneumocytes enabling gas exchange, and basal cells, the progenitor of the overlying epithelium (Rackley, C. R. & Stripp, B. R., The Journal of Clinical Investigation 122, 2724-2730 (2012)).
The protective role of the airway epithelium depends on a highly effective defence provided by airway mucus. Excessive mucus or impaired mucus clearance contributes to the pathogenesis of many common and less common airways diseases. The accumulation of mucus results from some combination of overproduction and decreased clearance and persistent accumulation can lead to infection, inflammation and suitable conditions for microbial growth (Fahy J. V. & Dickey B. F., The New England Journal of Medicine, 363, 2233-2247 (2010)).
Mucus hyper-secretion has been suggested to be an important pathological feature of chronic obstructive pulmonary disease (COPD) (Prescott, E. I. et al., 158, 6456 (1996)), asthma (Aikawa, T. et al., T. Chest, 101, 916 (1992)) and cystic fibrosis (Boucher, R. C. Adv. Drug Deliv. Rev., 54, 1359 (2002)). Substantial epidemiological studies have demonstrated an association between mucus hyper-secretion and increased frequency and duration of respiratory infection, hospitalization, increased levels of morbidity and mortality. It is presently clearly recognised that a hallmark of many airways diseases is an overabundance of goblet cells, referred to as mucus hypersecretory phenotype.
Mucus and the cells that produce it form an integral part of the mucociliary clearance apparatus whose co-ordinated function is considered essential to the protection of the airways from irritant/infectious insult. In healthy airways, secreted mucin is cleared by mucociliary clearance, infrequently assisted by conscious coughing, and does not therefore accumulate. In contrast, plugging of the small airways with mucus together with chronic cough and phlegm production are common manifestations in respiratory diseases.
Small airway plugging with mucus is a major contributing factor to fatal asthma, especially in younger patients (Kuyper, L. M. et al.; Am. J. Med., 115, (2003)), and is associated with goblet cell hyperplasia (Aikawa, T. et al., T. Chest, 101, 916 (1992)). In addition, 20% of asthmatics report chronic cough and phlegm production (Cerveri, S. et al.; Eur Respir J 22: 413-417 (2003)) and highlights the poorly controlled population (de Marco R. et al.; Am J Respir Crit Care Med 175, 32-39 (2007)). These patients are also those with significantly more upper and lower respiratory symptoms (Timonen K. L. et al.; Eur Respir J. March 19, 479-86 (2002)). Importantly, chronic mucus hypersecretion has also been identified as a significant marker of an enhanced decline in lung function in asthmatics (Lange P. et al.; N Engl J Med. 339, 1194-2000 (1998); Ulrik C. S. & Lange P. Am J Respir Crit Care Med. 150, 629-34 (1994)).
Goblet cells, which in healthy subjects are restricted to large airways, have been shown to be increased in numbers in small airways of patients affected by COPD and other respiratory diseases (Hogg J. C. Novartis Foundation Symposium, 234, 4 (2001). Whether goblet cell numbers increase as a consequence of progenitor cell proliferation (hyperplasia), non-mitotic differentiation (metaplasia) or inhibition of necrotic/apoptotic processes or whether one process predominates over the other is still poorly understood.
In the past decades, scientists have been elucidating molecules and pathways that regulate cell fate decisions. Most of these molecules operate in multiple tissues, at different stages of development or disease stage. Notch signaling is an evolutionarily conserved pathway that regulates many cell-fate decisions during development (Fortini, M. E., Dev Cell 16, 633-647 (2009)). Notch signaling in regulating cell fate decisions during development has been studied in many contexts, including in mucociliary tissues. There are 4 Notch receptors in mammalian cells (Notch1-4), which are activated by membrane-bound ligands, members of the Delta and Jagged family, on neighbouring cells. Notch activation leads to a series of cleavage events, culminating in the generation of the Notch intra-cellular domain (NICD), which translocates to the nucleus where it interacts with a transcription factor complex to regulate gene expression. In the epidermis of the Xenopus embryo, activation of Notch suppresses the ciliated cell fate, while inhibition of Notch signaling results in an overproduction of ciliated cells (Deblandre, G. A., et al., Development 126, 4715-4728 (1999)). In the developing mouse airway, expression of the NICD results in an overproduction of secretory cells at the expense of ciliated cells (Guseh, J. S., et al. Development 136, 1751-1759 (2009)), while deletion of Pofut1, an O-fucosyltransferase required for Notch-ligand interactions (Stahl, M., et al., The Journal of biological chemistry 283, 13638-13651 (2008)), or Rbpjk, a core nuclear effector of Notch signalling (Fortini, M. E., Dev Cell 16, 633-647 (2009)), results in an increase in the number of ciliated cells and a near absence of secretory cells (Tsao, P. N., et al., Development 136, 2297-2307 (2009)). A recent study using Notch receptor-specific knockouts suggested that Notch2 is the critical Notch receptor regulating secretory versus ciliated cell fate in the mouse developing airway (Morimoto, M. et al., Development 139, 4365-4373 (2012)). Albeit these recent investigations in the mouse developmental field, the role of Notch2 in the adult lung is not yet understood.
Hence, there is still the need for a greater understanding of the pathways that regulate the function of the human airway epithelium as well as those which define repair and remodelling in both health and disease. Identifying therapeutic targets in these pathways can lead to therapeutics for treating unmet medical needs such as moderate and severe asthma, cystic fibrosis and chronic obstructive pulmonary disease (COPD).